In many conventional myoelectric arm prostheses with a powered elbow joint, the configuration of the powered elbow joint is typically controlled irrespective of the configuration of the shoulder joint, and the wrist joint is typically controlled irrespective of the configuration of the other major joints in the arm. Further, in such myoelectric arm prostheses, the configuration of each joint in the prosthetic arm is typically controlled in a sequential manner (i.e., one joint is moved at a time), until the arm is configured in a desired posture.
The reason that conventional myoelectric prostheses are typically configured in a sequential manner is that the at least one joint of the myoelectric arm prosthesis is typically controlled with input from a pair of electromyogram (EMG) measurements, each of which measures the electrical activity resulting from a muscle contraction in a pair of opposing muscles in the residual limb of the amputated arm. In the case of an above-elbow amputation, the EMG is typically measured from the biceps and triceps muscle groups. In the anatomical arm, the combination of the pair of measurements provides a single bidirectional control for flexing or extending a given joint. Although the biceps and triceps muscles no longer act to flex and extend an anatomical elbow joint in above-elbow amputees, filtered and rectified EMG measurements can provide a real-time electrical signal proportional to the strength of contraction of the respective muscles in the residual limb. In most myoelectric arm prostheses, this electrical signal drives an elbow motor control loop for the prosthesis, such that the angular velocity of the prosthetic elbow joint is proportional to the measured EMG, where EMG is measured from the biceps muscle group for flexion and EMG is measured from the triceps muscle group for extension. Therefore, the angular speed of the elbow joint is generally proportional to the strength of contraction. In the absence of muscle contraction, the elbow remains locked at its current position (i.e., the joints of a myoelectric prosthetic arm are normally-locked). This control method can be referred to as differential, velocity-based EMG control.
In the case of an arm prosthesis with powered elbow and wrist joints, coordinated control of these joints is typically not possible since only a single control signal (from the biceps and triceps) is available for controlling these multiple joints. As a result, the EMG control signal is typically switched between joints, resulting in the joints being controlled in a sequential fashion. For example, the EMG from biceps/triceps will first be used to move the prosthetic elbow joint. Thereafter, control may be switched to the wrist joint (typically by momentarily co-contracting both the biceps and triceps in unison, rather than using them in a differential sense) and the EMG from the biceps/triceps can adjust the angle of the wrist. A further momentary co-contraction will switch control to a prosthetic hand, such that the same EMG will open/close the hand. A subsequent momentary co-contraction will cycle EMG control back to the elbow joint. As such, the same agonist/antagonist muscle group (in this case biceps and triceps) provide myoelectric control of all of the joints and the hand of the prosthetic arm.
With respect to coordinated control of the powered elbow joint and the intact shoulder joint in an above-elbow prosthesis, the powered elbow unit could be moved in concert with the shoulder, but not without great difficulty. In particular, since substantial portions of both the biceps and triceps extend over the shoulder joint (i.e., both are two-joint muscle groups), movement of the intact shoulder also produces myoelectric signals in the biceps and triceps. Therefore it is difficult, if not impossible, particularly without elbow proprioception, for the amputee to provide EMG signals to control the prosthetic elbow joint that are decoupled from EMG signals generated during use of the intact shoulder joint. As a result, the amputee typically resorts to the independent, sequential configuring of the shoulder and elbow joints, which removes the interference that shoulder movement presents in the biceps and triceps EMG signals. Therefore, above-elbow amputees are typically forced to move all joints of the arm in a sequential manner.